Various fibre optic based acoustic sensors are known. Many such sensors uses complex interferometry techniques and/or fibre optic point sensors or reflection sites arranged along the length of a fibre to provide acoustic sensing.
Recently the use of intrinsic scattering from a continuous length of fibre optic cable has been proposed, i.e. using a standard fibre optic cable without deliberately introduced reflection sites such a fibre Bragg grating or the like. GB patent application publication No. 2,442,745 describes a system wherein acoustic vibrations are sensed by launching a plurality of groups of pulse modulated electromagnetic waves into a standard optical fibre. The frequency of one pulse within a group differs from the frequency of another pulse in the group. The Rayleigh backscattering of light from intrinsic reflection sites within the fibre is sampled and demodulated at the frequency difference between the pulses in a group. N different analysis bins are used, each associated with a specific length of the fibre, and an acoustic disturbance in an individual analysis bin can be detected.
Use of such fibre distributed acoustic sensing systems has been proposed for perimeter monitoring or monitoring of oil or gas transmission pipelines. Generally however such acoustic systems are known for detecting an acoustic event in a particular analysis bin, for example for indicating that the perimeter being monitored has been crossed at a particular point.
It is an object of the present invention to provide new applications of fibre distributed acoustic sensing systems and methods and apparatus relating to fibre distributed acoustic sensing for use in such applications.